Allows intransitive verbs (i.e. verb derivations of base mods or base nouns) to take an in argument. The patient (value of the in argument) is the entity that experiences being in the state described by the intransitive verb. This happening is caused by the agent or subject of the action.
Because the point of lun is to enable an in argument for intransitive verbs (verb derivations of base mods or base nouns), it cannot be used on base verbs as they can already take an in argument (e.g. le lun makan, le lun kota, le lun amo, etc. are all disallowed); le pon ta le X can still be used to achieve causative constructions for base verbs.
This particle brings intransitive verbs up to the level of base verbs so that they can both be treated similarly:
Only links to the one content word after it, so adverbs can still follow: